1. Field of the Invention
The present invention relates to a holding device of a bearing and, more particularly, to an improvement of a holding device made of a plastic material for ball bearing or roller bearing for a finite rectilinear motion.
2. Description of the Prior Art
Recently, a number of rolling bearings for the finite rectilinear motion are used. In association with decrease in size and weight of the apparatus using bearings, the bearing itself becomes small-sized and light weight as well. To reduce the inertial force and weight and the like of the bearing, there occurs a necessity to use the holding device made of a light and easy-to-work plastic material. However, in this case, there is the problem in that the slide resistance is large.
According to a conventional method of supporting rolling members by the small-sized plastic holding device, there is a slight gap between each rolling member and the holding device in the state in which the holding device is in contact with the track member (This holding state is hereinafter referred to as a track member holding state).
To reduce the excessive slide resistance in the above constitution, the inventor of the present application has performed various kinds of experiments and analyzed the causes of such large slide resistance. Thus, the inventor has found out that the main cause relates to the method of supporting the rolling members of the plastic holding device.
Namely, the flat surfaces at which the track grooves of the table and bed are formed come into contact with almost the whole side surfaces of the holding device, so that the slide resistance therebetween causes the excessive slide resistance of the bearing. In particular, this tendency is remarkable in the case of grease lubrication. Consequently, the bearing using the plastic holding device cannot be used at the locations where a measuring instrument or the like is used in which it is required to minimize the slide resistance.
It is generally desirable to avoid contact between the track member and the holding device of the rolling bearing for the finite rectilinear motion (hereinafter, this ideal state is referred to as a rolling member holding state).
In the rolling member holding state, the rolling member comes into contact with the holding device in the state in which there are gaps among the track member and the upper and lower ends of the holding device (both end portions in the direction normal to the moving direction of the bearing), so that the ball supporting portion of the holding device does not come into contact with the track member. Therefore, as the contact between the holding device and the track member, only the upper and lower ends of the holding device come into contact with the flat surfaces at which the track groove is formed. Thus, the slide friction between the holding device and the track member decreases, resulting in a reduction of the slide resistance of the bearing.
However, to realize the rolling member holding state of the holding device, the outside dimensions of the window of the holding device must be set to be smaller than the inside dimensions thereof and the rolling member must be supported.
According to the conventional ball bearing technology, barrel-shaped pins, are arranged like cores in the injection molding die for the holding device and a plastic material is injected and molded at a high pressure and at a high temperature and thereafter, all of the barrel-shaped pins are pulled out in the same direction. However, in this method, the outside portion of the window locating in the pull-out direction of the pins is forcedly reamed and becomes larger than the outside dimensions of the window on the opposite side, so that there is a drawback such that a predetermined rolling member holding state cannot be derived.
As a method of solving such a drawback, the applicant of this application has already proposed the holding device having an I-shaped cross section such as to avoid the contact between the holding device and the track member as shown in FIG. 1 (Japanese Utility Model Application No. 141570/1984). Namely, FIG. 1 is a perspective view with a part cut away showing the conventional example. A holding device 4' is formed so as to have the I-shaped cross section, thereby reducing the slide resistance as small as possible. However, there is a drawback such that the shape of the molding die becomes complicated and the working cost is raised.